Attempts have been made to produce formed articles that are biodegradable from various types of proteinaceous material. An example of one such protein material is soy protein, particularly in the form of soy isolates and soy concentrates. Although soy protein has shown excellent plastic properties, this material does not possess a level of flowability that enables it to be readily processed by conventional equipment. As a result, soy-protein plastics have been traditionally made by compression molding.
Other efforts toward improving the processing of soy protein include treating the protein with a reducing agent to enhance the dispersibility of the protein in the mixture. Such agents function to reduce (cleave) disulfide (--S--S--) bonds in the protein molecule or polypeptide to form --SH groups, thus increasing the dispersibility of the protein. In one such example, U.S. Pat. No. 3,615,715 to Mullen, the authors describe forming films by the extrusion of a mixture made of a nonheat coagulable simple protein (i.e., wheat gluten, keratin, soybean protein), a reducing agent (i.e., an alkali metal sulfite), and particulate silica or silicate. Such mixtures include a high level of plasticizer to reduce the viscosity of the material and facilitate processing and extrusion.
Although useful in making films by extrusion, these mixtures do not have the structure or strength for forming solid articles by conventional thermoplastic processing techniques such as extrusion and injection molding.
Therefore, an object of the invention is to provide a thermoplastic material made of soybean protein that possesses a high level of flowability for processing the material into molded, solid plastic articles by extrusion and injection molding, and other like molding processes. Another object is to provide a soy protein thermoplastic composition that can be formed into solid, biodegradable articles having an increased level of tensile strength and elongation. Yet another object is to provide articles formed of a soy protein composition that are highly water-resistant, being capable of tolerating exposure to water over an extended period of time with minimal or no disintegration. Another object is to provide a low-cost biodegradable plastic for use in making formed articles.